The present invention relates to an extrusion die to be used for extruding a honeycomb structure.
FIG. 1 shows a catalyst support for purifying exhaust gas, which is a typical example of the honeycomb structure.
A catalyst support 1 is provided with a large number of open passages 12 which are surrounded by thin partition wall 11. Since the honeycomb structure having the above described construction is made of fragile material, mechanical strength thereof is low.
Therefore, the honeycomb structured catalyst support which is mounted in a vehicle, is in danger of being damaged due to thermal shock caused by rapid changes in exhaust gas temeprature, and vibrations of the running vehicle.
And the honeycomb structured catalyst support is also in danger of being damaged during the operation for mounting it in the vehicle.
In order to improve the mechanical strength, an outer peripheral wall 13 thicker than the partition wall 11 is formed in the outer peripheral portion of the honeycomb structure so as to be one body with the partition wall 11.
The conventional extrusion die for extruding the honeycomb structure provided with such an outer peripheral wall 13, has a construction shown in FIG. 2 or FIG. 3.
The extrusion die shown in FIG. 2 is provided with a die body 2 and a die mask 3. The die body 2 has a well known construction. In the outlet side of the extrusion die, interconnected grid-shaped extrusion slots 21 having the cross sectional shape corresponding to the partition wall 11, are formed.
In the inlet side of the extrusion die, a plurality of disconnected feed passageways 22 are formed. Each one end of the feed passageways 22 is interconnected with each one end of the extrusion slots within the die body 2.
In the outer peripheral portion of the end surface of the die body 2 on the outlet side thereof, the die mask 3 is disposed. Along the inner surface 31 of the die mask 3, the outer peripheral surface of the honeycomb structure which was extruded out of the die body 2 is formed. The diameter of the inner surface 31 of the die mask 3 is made smaller than that of the end surface of the die body 2, wherein the extrusion slots are formed. Between the end surface of the die body 2 and that of the die mask 3, an opening portion 4 is annularly formed.
The extrudable material is pressed into the extrusion slots 21 from the feed passageways 22 and is extruded out of the extrusion slots 21.
In this case, the material extruded into the annular opening portion 4 moves toward the central portion of the extrusion die to crush the open passages formed in the outer peripheral portion of the honeycomb structure which has been extruded out of the die body 2. Consequently, a honeycomb structure provided with a thick peripheral wall is obtained.
However, the material extruded into the opening portion 4 applies pressure into the outer periphery of the honeycomb structure which has been already extruded out of the die body 2 so that distortion is generated in the partition wall of the outer peripheral portion of the obtained honeycomb structure. Due to this distortion, strength of the honeycomb structure becomes nonuniform. Desirable mechanical strength cannot be obtained throughout the obtained honeycomb structure.
In the extrusion die shown in FIG. 3, the outer periphery of the end surface of the die body 2 on the outlet side thereof, is formed into a tapered surface inclined at an angle of .THETA..sub.1, and the end surface of the die mask 3, which is opposed to the tapered surface of the die body 2, is also formed into a tapered surface inclined at an angle of .THETA..sub.2 to the tapered surface of the die body 2. An annular opening portion 4 is formed between the above two tapered surfaces.
In this extrusion die shown in FIG. 3, since the moving direction of the material extruded into the opening portion 4 approaches to that of the material which is extruded out of the extrusion slots positioned in the central portion of the die body 2 straightly, the pressure applied to the outer peripheral portion of the extruded honeycomb structure is slightly weakened so that the generation of the distortion is also decreased.
However, in the extrusion die shown in FIG. 3, the generation of the distortion cannot be prevented completely since pressure is applied to the outer periphery of the honeycomb structure after being extruded.
In particular, when a honeycomb structure having a partition wall with a thickness of about 0.3 mm or less is formed by the conventional extrusion die, distortion is frequently generated due to the pressure applied by the material extruded into the opening portion 4. As a result, there may be a case when the honeycomb structure cannot be extruded out of the conventional die.
Accordingly, one object of the present invention is to provide an extrusion die for extruding a honeycomb structure provided with a thick peripheral wall and having an excellent mechanical strength.
Another object of the present invention is to provide an extrusion die by which a thick peripheral wall can be integrally formed in the outer peripheral portion of a honeycomb structure without generating any distortion therein.
Still another object of the present invention is to provide an extrusion die suitable for extruding a catalyst support for purifying exhaust gas, provided with a large number of narrow open passages which are divided by a thin partition wall and a thick peripheral wall which is integrally formed on the outer periphery thereof, and having excellent strength.